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          高级类型
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          (adsbygoogle = window.adsbygoogle || []).push({});
      </script></div> <div class="theme-vdoing-content content__default"><h1 id="高级类型"><a href="#高级类型" class="header-anchor">#</a> 高级类型</h1> <h2 id="交叉类型"><a href="#交叉类型" class="header-anchor">#</a> 交叉类型</h2> <p>交叉类型是将多个类型合并为一个类型。 这让我们可以把现有的多种类型叠加到一起成为一种类型，它包含了所需的所有类型的特性。 例如，<code>Person &amp; Loggable</code> 同时是 <code>Person</code> 和 <code>Loggable</code>。 就是说这个类型的对象同时拥有了这两种类型的成员。</p> <p>我们大多是在混入（mixins）或其它不适合典型面向对象模型的地方看到交叉类型的使用。 （在 JavaScript 里发生这种情况的场合很多！） 下面是如何创建混入的一个简单例子：</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">function</span> <span class="token generic-function"><span class="token function">extend</span><span class="token generic class-name"><span class="token operator">&lt;</span><span class="token constant">T</span><span class="token punctuation">,</span> <span class="token constant">U</span><span class="token operator">&gt;</span></span></span> <span class="token punctuation">(</span>first<span class="token operator">:</span> <span class="token constant">T</span><span class="token punctuation">,</span> second<span class="token operator">:</span> <span class="token constant">U</span><span class="token punctuation">)</span><span class="token operator">:</span> <span class="token constant">T</span> <span class="token operator">&amp;</span> <span class="token constant">U</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> result <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span> <span class="token keyword">as</span> <span class="token constant">T</span> <span class="token operator">&amp;</span> <span class="token constant">U</span>
  <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> id <span class="token keyword">in</span> first<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    result<span class="token punctuation">[</span>id<span class="token punctuation">]</span> <span class="token operator">=</span> first<span class="token punctuation">[</span>id<span class="token punctuation">]</span> <span class="token keyword">as</span> <span class="token builtin">any</span>
  <span class="token punctuation">}</span>
  <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> id <span class="token keyword">in</span> second<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>result<span class="token punctuation">.</span><span class="token function">hasOwnProperty</span><span class="token punctuation">(</span>id<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      result<span class="token punctuation">[</span>id<span class="token punctuation">]</span> <span class="token operator">=</span> second<span class="token punctuation">[</span>id<span class="token punctuation">]</span> <span class="token keyword">as</span> <span class="token builtin">any</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span>
  <span class="token keyword">return</span> result
<span class="token punctuation">}</span>

<span class="token keyword">class</span> <span class="token class-name">Person</span> <span class="token punctuation">{</span>
  <span class="token function">constructor</span> <span class="token punctuation">(</span><span class="token keyword">public</span> name<span class="token operator">:</span> <span class="token builtin">string</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token keyword">interface</span> <span class="token class-name">Loggable</span> <span class="token punctuation">{</span>
  <span class="token function">log</span> <span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">:</span> <span class="token keyword">void</span>
<span class="token punctuation">}</span>

<span class="token keyword">class</span> <span class="token class-name">ConsoleLogger</span> <span class="token keyword">implements</span> <span class="token class-name">Loggable</span> <span class="token punctuation">{</span>
  <span class="token function">log</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment">// ...</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token keyword">var</span> jim <span class="token operator">=</span> <span class="token function">extend</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token string">'Jim'</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token keyword">new</span> <span class="token class-name">ConsoleLogger</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
<span class="token keyword">var</span> n <span class="token operator">=</span> jim<span class="token punctuation">.</span>name
jim<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br><span class="line-number">31</span><br></div></div><h2 id="联合类型"><a href="#联合类型" class="header-anchor">#</a> 联合类型</h2> <p>联合类型与交叉类型很有关联，但是使用上却完全不同。 偶尔你会遇到这种情况，一个代码库希望传入 <code>number</code> 或 <code>string</code> 类型的参数。 例如下面的函数：</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">function</span> <span class="token function">padLeft</span><span class="token punctuation">(</span>value<span class="token operator">:</span> <span class="token builtin">string</span><span class="token punctuation">,</span> padding<span class="token operator">:</span> <span class="token builtin">any</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> padding <span class="token operator">===</span> <span class="token string">'number'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token function">Array</span><span class="token punctuation">(</span>padding <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token string">' '</span><span class="token punctuation">)</span> <span class="token operator">+</span> value
  <span class="token punctuation">}</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> padding <span class="token operator">===</span> <span class="token string">'string'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> padding <span class="token operator">+</span> value
  <span class="token punctuation">}</span>
  <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">Expected string or number, got '</span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>padding<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string">'.</span><span class="token template-punctuation string">`</span></span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>

<span class="token function">padLeft</span><span class="token punctuation">(</span><span class="token string">'Hello world'</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">)</span> <span class="token comment">// returns &quot;    Hello world&quot;</span>

</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p><code>padLeft</code> 存在一个问题，<code>padding</code> 参数的类型指定成了 <code>any</code>。 这就是说我们可以传入一个既不是 <code>number</code> 也不是 <code>string</code> 类型的参数，但是 TypeScript 却不报错。</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">let</span> indentedString <span class="token operator">=</span> <span class="token function">padLeft</span><span class="token punctuation">(</span><span class="token string">'Hello world'</span><span class="token punctuation">,</span> <span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token comment">// 编译阶段通过，运行时报错</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>为了解决这个问题，我们可以使用 联合类型做为 <code>padding</code> 的参数：</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">function</span> <span class="token function">padLeft</span><span class="token punctuation">(</span>value<span class="token operator">:</span> <span class="token builtin">string</span><span class="token punctuation">,</span> padding<span class="token operator">:</span> <span class="token builtin">string</span> <span class="token operator">|</span> <span class="token builtin">number</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// ...</span>
<span class="token punctuation">}</span>

<span class="token keyword">let</span> indentedString <span class="token operator">=</span> <span class="token function">padLeft</span><span class="token punctuation">(</span><span class="token string">'Hello world'</span><span class="token punctuation">,</span> <span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token comment">// 编译阶段报错</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>联合类型表示一个值可以是几种类型之一。我们用竖线（<code>|</code>）分隔每个类型，所以 <code>number | string</code> 表示一个值可以是 <code>number</code> 或 <code>string</code>。</p> <p>如果一个值是联合类型，我们只能访问此联合类型的所有类型里共有的成员。</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">interface</span> <span class="token class-name">Bird</span> <span class="token punctuation">{</span>
  <span class="token function">fly</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
  <span class="token function">layEggs</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>

<span class="token keyword">interface</span> <span class="token class-name">Fish</span> <span class="token punctuation">{</span>
  <span class="token function">swim</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
  <span class="token function">layEggs</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>

<span class="token keyword">function</span> <span class="token function">getSmallPet</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">:</span> Fish <span class="token operator">|</span> Bird <span class="token punctuation">{</span>
  <span class="token comment">// ...</span>
<span class="token punctuation">}</span>

<span class="token keyword">let</span> pet <span class="token operator">=</span> <span class="token function">getSmallPet</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
pet<span class="token punctuation">.</span><span class="token function">layEggs</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// okay</span>
pet<span class="token punctuation">.</span><span class="token function">swim</span><span class="token punctuation">(</span><span class="token punctuation">)</span>    <span class="token comment">// error</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br></div></div><p>这里的联合类型可能有点复杂：如果一个值的类型是 <code>A | B</code>，我们能够确定的是它包含了 <code>A</code> 和 <code>B</code> 中共有的成员。这个例子里，<code>Fish</code> 具有一个 <code>swim</code> 方法，我们不能确定一个 <code>Bird | Fish</code> 类型的变量是否有 <code>swim</code>方法。 如果变量在运行时是 <code>Bird</code> 类型，那么调用 <code>pet.swim()</code> 就出错了。</p> <h2 id="类型保护"><a href="#类型保护" class="header-anchor">#</a> 类型保护</h2> <p>联合类型适合于那些值可以为不同类型的情况。 但当我们想确切地了解是否为 <code>Fish</code> 或者是 <code>Bird</code> 时怎么办？ JavaScript 里常用来区分这 2 个可能值的方法是检查成员是否存在。如之前提及的，我们只能访问联合类型中共同拥有的成员。</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">let</span> pet <span class="token operator">=</span> <span class="token function">getSmallPet</span><span class="token punctuation">(</span><span class="token punctuation">)</span>

<span class="token comment">// 每一个成员访问都会报错</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>pet<span class="token punctuation">.</span>swim<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  pet<span class="token punctuation">.</span><span class="token function">swim</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>pet<span class="token punctuation">.</span>fly<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  pet<span class="token punctuation">.</span><span class="token function">fly</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>为了让这段代码工作，我们要使用类型断言：</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">let</span> pet <span class="token operator">=</span> <span class="token function">getSmallPet</span><span class="token punctuation">(</span><span class="token punctuation">)</span>

<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>pet <span class="token keyword">as</span> Fish<span class="token punctuation">)</span><span class="token punctuation">.</span>swim<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token punctuation">(</span>pet <span class="token keyword">as</span> Fish<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">swim</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
  <span class="token punctuation">(</span>pet <span class="token keyword">as</span> Bird<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">fly</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><h3 id="用户自定义的类型保护"><a href="#用户自定义的类型保护" class="header-anchor">#</a> 用户自定义的类型保护</h3> <p>这里可以注意到我们不得不多次使用类型断言。如果我们一旦检查过类型，就能在之后的每个分支里清楚地知道 <code>pet</code> 的类型的话就好了。</p> <p>TypeScript 里的<em>类型保护</em>机制让它成为了现实。 类型保护就是一些表达式，它们会在运行时检查以确保在某个作用域里的类型。定义一个类型保护，我们只要简单地定义一个函数，它的返回值是一个<em>类型谓词</em>：</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">function</span> <span class="token function">isFish</span><span class="token punctuation">(</span>pet<span class="token operator">:</span> Fish <span class="token operator">|</span> Bird<span class="token punctuation">)</span><span class="token operator">:</span> pet <span class="token keyword">is</span> Fish <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token punctuation">(</span>pet <span class="token keyword">as</span> Fish<span class="token punctuation">)</span><span class="token punctuation">.</span>swim <span class="token operator">!==</span> <span class="token keyword">undefined</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>在这个例子里，<code>pet is Fish</code> 就是类型谓词。谓词为 <code>parameterName is Type</code> 这种形式， <code>parameterName</code> 必须是来自于当前函数签名里的一个参数名。</p> <p>每当使用一些变量调用 <code>isFish</code> 时，<code>TypeScript</code> 会将变量缩减为那个具体的类型。</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">isFish</span><span class="token punctuation">(</span>pet<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  pet<span class="token punctuation">.</span><span class="token function">swim</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>
<span class="token keyword">else</span> <span class="token punctuation">{</span>
  pet<span class="token punctuation">.</span><span class="token function">fly</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>注意 <code>TypeScript</code> 不仅知道在 <code>if</code> 分支里 <code>pet</code> 是 <code>Fish</code> 类型；它还清楚在 <code>else</code> 分支里，一定不是 Fish类型而是 <code>Bird</code> 类型。</p> <h3 id="typeof-类型保护"><a href="#typeof-类型保护" class="header-anchor">#</a> typeof 类型保护</h3> <p>现在我们回过头来看看怎么使用联合类型书写 <code>padLeft</code> 代码。我们可以像下面这样利用类型断言来写：</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">function</span> <span class="token function">isNumber</span> <span class="token punctuation">(</span>x<span class="token operator">:</span> <span class="token builtin">any</span><span class="token punctuation">)</span><span class="token operator">:</span>x <span class="token keyword">is</span> <span class="token builtin">string</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token keyword">typeof</span> x <span class="token operator">===</span> <span class="token string">'number'</span>
<span class="token punctuation">}</span>

<span class="token keyword">function</span> <span class="token function">isString</span> <span class="token punctuation">(</span>x<span class="token operator">:</span> <span class="token builtin">any</span><span class="token punctuation">)</span><span class="token operator">:</span> x <span class="token keyword">is</span> <span class="token builtin">string</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token keyword">typeof</span> x <span class="token operator">===</span> <span class="token string">'string'</span>
<span class="token punctuation">}</span>

<span class="token keyword">function</span> <span class="token function">padLeft</span> <span class="token punctuation">(</span>value<span class="token operator">:</span> <span class="token builtin">string</span><span class="token punctuation">,</span> padding<span class="token operator">:</span> <span class="token builtin">string</span> <span class="token operator">|</span> <span class="token builtin">number</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">isNumber</span><span class="token punctuation">(</span>padding<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token function">Array</span><span class="token punctuation">(</span>padding <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token string">' '</span><span class="token punctuation">)</span> <span class="token operator">+</span> value
  <span class="token punctuation">}</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">isString</span><span class="token punctuation">(</span>padding<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> padding <span class="token operator">+</span> value
  <span class="token punctuation">}</span>
  <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">Expected string or number, got '</span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>padding<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string">'.</span><span class="token template-punctuation string">`</span></span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br></div></div><p>然而，你必须要定义一个函数来判断类型是否是原始类型，但这并不必要。其实我们不必将 <code>typeof x === 'number'</code>抽象成一个函数，因为 TypeScript 可以将它识别为一个类型保护。 也就是说我们可以直接在代码里检查类型了。</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">function</span> <span class="token function">padLeft</span> <span class="token punctuation">(</span>value<span class="token operator">:</span> <span class="token builtin">string</span><span class="token punctuation">,</span> padding<span class="token operator">:</span> <span class="token builtin">string</span> <span class="token operator">|</span> <span class="token builtin">number</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> padding <span class="token operator">===</span> <span class="token string">'number'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token function">Array</span><span class="token punctuation">(</span>padding <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token string">' '</span><span class="token punctuation">)</span> <span class="token operator">+</span> value
  <span class="token punctuation">}</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> padding <span class="token operator">===</span> <span class="token string">'string'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> padding <span class="token operator">+</span> value
  <span class="token punctuation">}</span>
  <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">Expected string or number, got '</span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>padding<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string">'.</span><span class="token template-punctuation string">`</span></span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>这些 <code>typeof</code> 类型保护只有两种形式能被识别：<code>typeof v === &quot;typename&quot;</code> 和 <code>typeof v !== &quot;typename&quot;</code>， <code>&quot;typename&quot;</code>必须是 <code>&quot;number&quot;</code>， <code>&quot;string&quot;</code>，<code>&quot;boolean&quot;</code> 或 <code>&quot;symbol&quot;</code>。 但是 TypeScript 并不会阻止你与其它字符串比较，只是 TypeScript 不会把那些表达式识别为类型保护。</p> <h3 id="instanceof-类型保护"><a href="#instanceof-类型保护" class="header-anchor">#</a> instanceof 类型保护</h3> <p>如果你已经阅读了 <code>typeof</code> 类型保护并且对 JavaScript 里的 <code>instanceof</code> 操作符熟悉的话，你可能已经猜到了这节要讲的内容。</p> <p><code>instanceof</code> 类型保护是通过构造函数来细化类型的一种方式。我们把之前的例子做一个小小的改造：</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">class</span> <span class="token class-name">Bird</span> <span class="token punctuation">{</span>
  <span class="token function">fly</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token builtin">console</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'bird fly'</span><span class="token punctuation">)</span>
  <span class="token punctuation">}</span>

  <span class="token function">layEggs</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token builtin">console</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'bird lay eggs'</span><span class="token punctuation">)</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token keyword">class</span> <span class="token class-name">Fish</span> <span class="token punctuation">{</span>
  <span class="token function">swim</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token builtin">console</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'fish swim'</span><span class="token punctuation">)</span>
  <span class="token punctuation">}</span>

  <span class="token function">layEggs</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token builtin">console</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'fish lay eggs'</span><span class="token punctuation">)</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token keyword">function</span> <span class="token function">getRandomPet</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> Math<span class="token punctuation">.</span><span class="token function">random</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&gt;</span> <span class="token number">0.5</span> <span class="token operator">?</span> <span class="token keyword">new</span> <span class="token class-name">Bird</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">:</span> <span class="token keyword">new</span> <span class="token class-name">Fish</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>

<span class="token keyword">let</span> pet <span class="token operator">=</span> <span class="token function">getRandomPet</span><span class="token punctuation">(</span><span class="token punctuation">)</span>

<span class="token keyword">if</span> <span class="token punctuation">(</span>pet <span class="token keyword">instanceof</span> <span class="token class-name">Bird</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  pet<span class="token punctuation">.</span><span class="token function">fly</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>pet <span class="token keyword">instanceof</span> <span class="token class-name">Fish</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  pet<span class="token punctuation">.</span><span class="token function">swim</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br><span class="line-number">31</span><br><span class="line-number">32</span><br></div></div><h2 id="可以为-null-的类型"><a href="#可以为-null-的类型" class="header-anchor">#</a> 可以为 null 的类型</h2> <p>TypeScript 具有两种特殊的类型，<code>null</code> 和 <code>undefined</code>，它们分别具有值 <code>null</code> 和 <code>undefined</code>。我们在<a href="/chapter2/type">基础类型</a>一节里已经做过简要说明。 默认情况下，类型检查器认为 <code>null</code> 与 <code>undefined</code> 可以赋值给任何类型。 <code>null</code> 与 <code>undefined</code> 是所有其它类型的一个有效值。 这也意味着，你阻止不了将它们赋值给其它类型，就算是你想要阻止这种情况也不行。<code>null</code>的发明者，Tony Hoare，称它为<a href="https://en.wikipedia.org/wiki/Null_pointer#History" target="_blank" rel="noopener noreferrer">价值亿万美金的错误<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>。</p> <p><code>--strictNullChecks</code> 标记可以解决此错误：当你声明一个变量时，它不会自动地包含 <code>null</code> 或 <code>undefined</code>。 你可以使用联合类型明确的包含它们：</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">let</span> s <span class="token operator">=</span> <span class="token string">'foo'</span>
s <span class="token operator">=</span> <span class="token keyword">null</span> <span class="token comment">// 错误, 'null'不能赋值给'string'</span>
<span class="token keyword">let</span> sn<span class="token operator">:</span> <span class="token builtin">string</span> <span class="token operator">|</span> <span class="token keyword">null</span> <span class="token operator">=</span> <span class="token string">'bar'</span>
sn <span class="token operator">=</span> <span class="token keyword">null</span> <span class="token comment">// 可以</span>

sn <span class="token operator">=</span> <span class="token keyword">undefined</span> <span class="token comment">// error, 'undefined'不能赋值给'string | null'</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>注意，按照 JavaScript 的语义，TypeScript 会把 <code>null</code> 和 <code>undefined</code> 区别对待。<code>string | null</code>，<code>string | undefined</code> 和 <code>string | undefined | null</code> 是不同的类型。</p> <h3 id="可选参数和可选属性"><a href="#可选参数和可选属性" class="header-anchor">#</a> 可选参数和可选属性</h3> <p>使用了 <code>--strictNullChecks</code>，可选参数会被自动地加上 <code>| undefined</code>:</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span>x<span class="token operator">:</span> <span class="token builtin">number</span><span class="token punctuation">,</span> y<span class="token operator">?</span><span class="token operator">:</span> <span class="token builtin">number</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> x <span class="token operator">+</span> <span class="token punctuation">(</span>y <span class="token operator">||</span> <span class="token number">0</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>
<span class="token function">f</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span>
<span class="token function">f</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span>
<span class="token function">f</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token keyword">undefined</span><span class="token punctuation">)</span>
<span class="token function">f</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token comment">// error, 'null' 不能赋值给 'number | undefined'</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>可选属性也会有同样的处理：</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">class</span> <span class="token class-name"><span class="token constant">C</span></span> <span class="token punctuation">{</span>
  a<span class="token operator">:</span> <span class="token builtin">number</span>
  b<span class="token operator">?</span><span class="token operator">:</span> <span class="token builtin">number</span>
<span class="token punctuation">}</span>
<span class="token keyword">let</span> c <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name"><span class="token constant">C</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span>
c<span class="token punctuation">.</span>a <span class="token operator">=</span> <span class="token number">12</span>
c<span class="token punctuation">.</span>a <span class="token operator">=</span> <span class="token keyword">undefined</span> <span class="token comment">// error, 'undefined' 不能赋值给 'number'</span>
c<span class="token punctuation">.</span>b <span class="token operator">=</span> <span class="token number">13</span>
c<span class="token punctuation">.</span>b <span class="token operator">=</span> <span class="token keyword">undefined</span> <span class="token comment">// ok</span>
c<span class="token punctuation">.</span>b <span class="token operator">=</span> <span class="token keyword">null</span> <span class="token comment">// error, 'null' 不能赋值给 'number | undefined'</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><h3 id="类型保护和类型断言"><a href="#类型保护和类型断言" class="header-anchor">#</a> 类型保护和类型断言</h3> <p>由于可以为 <code>null</code> 的类型能和其它类型定义为联合类型，那么你需要使用类型保护来去除 <code>null</code>。幸运地是这与在 <code>JavaScript</code> 里写的代码一致：</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span>sn<span class="token operator">:</span> <span class="token builtin">string</span> <span class="token operator">|</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token operator">:</span> <span class="token builtin">string</span> <span class="token punctuation">{</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span>sn <span class="token operator">===</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token string">'default'</span>
  <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> sn
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>这里很明显地去除了 <code>null</code>，你也可以使用短路运算符：</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span>sn<span class="token operator">:</span> <span class="token builtin">string</span> <span class="token operator">|</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token operator">:</span> <span class="token builtin">string</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> sn <span class="token operator">||</span> <span class="token string">'default'</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>如果编译器不能够去除 <code>null</code> 或 <code>undefined</code>，你可以使用类型断言手动去除。语法是添加 <code>!</code> 后缀： <code>identifier!</code> 从 <code>identifier</code> 的类型里去除了 <code>null</code> 和 <code>undefined</code>：</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">function</span> <span class="token function">broken</span><span class="token punctuation">(</span>name<span class="token operator">:</span> <span class="token builtin">string</span> <span class="token operator">|</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token operator">:</span> <span class="token builtin">string</span> <span class="token punctuation">{</span>
  <span class="token keyword">function</span> <span class="token function">postfix</span><span class="token punctuation">(</span>epithet<span class="token operator">:</span> <span class="token builtin">string</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> name<span class="token punctuation">.</span><span class="token function">charAt</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">'.  the '</span> <span class="token operator">+</span> epithet <span class="token comment">// error, 'name' 可能为 null</span>
  <span class="token punctuation">}</span>
  name <span class="token operator">=</span> name <span class="token operator">||</span> <span class="token string">'Bob'</span>
  <span class="token keyword">return</span> <span class="token function">postfix</span><span class="token punctuation">(</span><span class="token string">'great'</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>

<span class="token keyword">function</span> <span class="token function">fixed</span><span class="token punctuation">(</span>name<span class="token operator">:</span> <span class="token builtin">string</span> <span class="token operator">|</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token operator">:</span> <span class="token builtin">string</span> <span class="token punctuation">{</span>
  <span class="token keyword">function</span> <span class="token function">postfix</span><span class="token punctuation">(</span>epithet<span class="token operator">:</span> <span class="token builtin">string</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> name<span class="token operator">!</span><span class="token punctuation">.</span><span class="token function">charAt</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">'.  the '</span> <span class="token operator">+</span> epithet <span class="token comment">// ok</span>
  <span class="token punctuation">}</span>
  name <span class="token operator">=</span> name <span class="token operator">||</span> <span class="token string">'Bob'</span>
  <span class="token keyword">return</span> <span class="token function">postfix</span><span class="token punctuation">(</span><span class="token string">'great'</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>

<span class="token function">broken</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span>

</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br></div></div><p>本例使用了嵌套函数，因为编译器无法去除嵌套函数的 <code>null</code>（除非是立即调用的函数表达式）。因为它无法跟踪所有对嵌套函数的调用，尤其是你将内层函数做为外层函数的返回值。如果无法知道函数在哪里被调用，就无法知道调用时 <code>name</code> 的类型。</p> <h2 id="字符串字面量类型"><a href="#字符串字面量类型" class="header-anchor">#</a> 字符串字面量类型</h2> <p>字符串字面量类型允许你指定字符串必须具有的确切值。在实际应用中，字符串字面量类型可以与联合类型，类型保护很好的配合。通过结合使用这些特性，你可以实现类似枚举类型的字符串。</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">type</span> <span class="token class-name">Easing</span> <span class="token operator">=</span> <span class="token string">'ease-in'</span> <span class="token operator">|</span> <span class="token string">'ease-out'</span> <span class="token operator">|</span> <span class="token string">'ease-in-out'</span>

<span class="token keyword">class</span> <span class="token class-name">UIElement</span> <span class="token punctuation">{</span>
  <span class="token function">animate</span> <span class="token punctuation">(</span>dx<span class="token operator">:</span> <span class="token builtin">number</span><span class="token punctuation">,</span> dy<span class="token operator">:</span> <span class="token builtin">number</span><span class="token punctuation">,</span> easing<span class="token operator">:</span> Easing<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>easing <span class="token operator">===</span> <span class="token string">'ease-in'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token comment">// ...</span>
    <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>easing <span class="token operator">===</span> <span class="token string">'ease-out'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>easing <span class="token operator">===</span> <span class="token string">'ease-in-out'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
      <span class="token comment">// error! 不能传入 null 或者 undefined.</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token keyword">let</span> button <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">UIElement</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
button<span class="token punctuation">.</span><span class="token function">animate</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token string">'ease-in'</span><span class="token punctuation">)</span>
button<span class="token punctuation">.</span><span class="token function">animate</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token string">'uneasy'</span><span class="token punctuation">)</span> <span class="token comment">// error</span>

</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br></div></div><p>你只能从三种允许的字符中选择其一来做为参数传递，传入其它值则会产生错误。</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>Argument of type '&quot;uneasy&quot;' is not assignable to parameter of type '&quot;ease-in&quot; | &quot;ease-out&quot; | &quot;ease-in-out&quot;'
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><h2 id="总结"><a href="#总结" class="header-anchor">#</a> 总结</h2> <p>那么到这里，我们的 TypeScript 常用语法学习就告一段落了，当然 TypeScript 还有其他的语法我们并没有讲，我们只是讲了 TypeScript 的一些常用语法，你们把这些知识学会已经足以开发一般的应用了。如果你在使用 TypeScript 开发项目中遇到了其他的 TypeScript 语法知识，你可以通过 TypeScript 的<a href="https://www.typescriptlang.org/docs/home.html" target="_blank" rel="noopener noreferrer">官网文档<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>学习。因为学基础最好的方法还是去阅读它的官网文档，敲上面的小例子。其实我们课程的基础知识结构也是大部分参考了官网文档，要记住学习一门技术的基础官网文档永远是最好的第一手资料。</p> <p>但是 TypeScript 的学习不能仅仅靠看官网文档，你还需要动手实践，在实践中你才能真正掌握 TypeScript。相信很多同学学习到这里已经迫不及待想要大展身手了，那么下面我们就开始把理论转换为实践，一起来用 TypeScript 重构 axios 吧！</p></div></div> <div class="page-slot page-slot-bottom"><!-- 横向自适应 -->
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